FIG. 1A depicts digital camera 100 capturing an image of document 101 according to the prior art. Light is reflected from document 101 and is received by optical subsystem 102. Optical subsystem 102 optically reduces the image of document 101 to focus the image on charge-coupled device (CCD) 103. CCD 103 is typically implemented as a two-dimensional array of photosensitive capacitive elements. When light is incident on the photosensitive elements of CCD 103, charge is trapped in a depletion region of the semiconductor material of the elements. The amount of charge associated with the photosensitive capacitive elements is related to the intensity of light incident on the respective elements received over a sampling period. Accordingly, the image is captured by determining the intensity of incident light at the respective photosensitive capacitive elements via sampling the elements.
The analog information produced by the photosensitive capacitive elements is converted to digital information by analog-to-digital (A/D) conversion unit 104. A/D conversion unit 104 may convert the analog information received from CCD 103 in either a serial or parallel manner. The converted digital information may be stored in memory 105 (e.g., random access memory). The digital information is then processed by processor 106 according to control software stored in ROM 107 (e.g., PROM, EPROM, EEPROM, and/or the like). For example, the digital information may be compressed according to the Joint Photographic Experts Group (JPEG) standard. Additionally or alternatively, other circuitry (not shown) may be utilized to process the captured image such as an application specific integrated circuit (ASIC). User interface 108 (e.g., a touch screen, keys, and/or the like) may be utilized to edit the captured and processed image. The image may then be provided to output port 109. For example, the user may cause the image to be downloaded to a personal computer (not shown) via output port 109.
The quality of the captured image is dependent on the perspective or positioning of digital camera 100 with respect to document 101. Specifically, if digital camera 100 is off-angle, the captured image of document 101 may be skewed as shown in captured image 150 of FIG. 1B. Therefore, off-angle positioning may appreciably reduce the readability of the captured image of document 101.
Accordingly, the image data may be uploaded to a personal computer for processing by various known correction algorithms. The algorithms are employed to correct the distortion effects associated with off-angle images of documents. Typical known correction algorithms require a user to manually identify the corners of a region of a captured image. By measuring the spatial displacement of the identified corners from desired positions associated with a rectangular arrangement, an estimation of the amount of distortion is calculated. The correction algorithm then processes the imaged document to possess the desired perspective and size as necessary and may produce perspective enhanced image 200 of FIG. 2.
An automatic corner detection algorithm is described by G. F. McLean in Geometric Correction of Digitized Art, GRAPHICAL MODELS AND IMAGE PROCESSING, Vol. 58, No. 2, March, pp. 142-154 (1996). McLean's algorithm is intended to correct the perspective distortion associated with “archival images of two-dimensional art objects.” Accordingly, the algorithm assumes that some degree of care was taken during the imaging or photography. Thus, the algorithm assumes that the resulting distorted quadrilaterals of the art form “a set of roughly 90° corners.” Upon this assumption, the corners may be estimated by analyzing the intersection of lines that form approximately 90° interior angles. Although the McLean algorithm is clearly advantageous as compared to pure manual selection of corners, the assumptions of this algorithm are not always appropriate for digital images (e.g., those taken by casual users) which may exhibit appreciable perspective distortion.